[S-CG67_1PM1] 海洋底地球科学

[SCG67-04] 広帯域海底地震探査による海洋リソスフェア/アセノスフェアシステムの地震波速度異方性構造推定

*竹尾 明子¹、一瀬 建日¹、西田 究¹、川勝 均¹、塩原 肇¹、杉岡 裕子²、伊藤 亜妃²、歌田 久司¹ (1.東京大学地震研究所、2.海洋開発機構地球内部ダイナミクス領域)

キーワード：表面波, 地震波干渉法, 異方性, プレートテクトニクス

The uppermost mantle structure beneath the oceanic basins is essential to discuss the oceanic lithosphere/asthenosphere system, the most simple and representative system of the theory of plate tectonics. Seismic anisotropy within the oceanic lithosphere and asthenosphere is especially important, as it reflects the flow and deformation in the uppermost mantle. Previous structural studies have been, however, limited in terms of the depth range: the top of lithosphere at depths of ~10-20 km by refraction surveys, and the structure deeper than ~30 km by surface-wave tomography studies. There has been no discussion from the top of the lithosphere continuously to the asthenosphere, which needs the broadband analysis of surface waves at periods of 3-100 s. In addition, there has been limited discussion about the intensity of seismic anisotropy because of the difficulty of estimating the absolute value of seismic anisotropy by surface-wave tomography studies.We have developed a new multi-band method to analyze surface waves in broadband array records of ocean bottom seismometers for determining seismic anisotropy structure at depths of ~10-100 km quantitatively (Takeo et al 2013 JGR, submitted to GJI). The method uses the ambient noise cross-correlation method for analyzing surface waves at periods of 3-30 s and to determine structure at depths shallower than ~50 km, as well as the array analysis method of teleseismic waveforms at longer periods for determining deeper structures. In previous studies, we have applied the multi-band method to records obtained in three oceanic regions: the Shikoku Basin region (Takeo et al. 2013 JGR), the southwestern region of the Shatsky Rise (Takeo et al. submitted to GJI) and the French Polynesia region (Takeo et al. 2012 SSJ Fall Meeting). In this study, we applied the same method to the records of broadband ocean bottom seismometers obtained by the normal oceanic mantle (NOMan) project at two oceanic regions (northwestern and southeastern regions of the Shatsky Rise) from 2010 to 2013. By combining the results for five oceanic regions with different seafloor ages between 20 and 155 Ma, we can discuss the seismic structure and its anisotropy in the oceanic lithosphere and asthenosphere, and the deformation of mantle related to plate motions.The results for five oceanic regions can be summarized into five points: (i) the high-velocity lid and the low velocity zone corresponds to the oceanic lithosphere and asthenosphere, (ii) the transition from the lithosphere to the asthenosphere occurs at depths of 40-90 km, (iii) the average intensity of S-wave radial anisotropy is 3-6 % at depths of 10-150 km with the velocity of horizontally propagating and vertically polarized S-wave slower than the horizontally polarized S-wave, (iv) the intensity of S-wave azimuthal anisotropy at depths of 10-100 km is weaker than that of S-wave radial anisotropy and weakens with depths, and (v) the azimuth of maximum S-wave velocity is not perpendicular to ancient spreading axis in general. These results indicate complex deformation system in the present and ancient oceanic asthenosphere related to the presence of partial melting, the unusual fabric of olivine and so on. We will summarize these results and discussions, and will also present the potential of the broadband ocean bottom seismology to elucidate structure and deformation in the oceanic lithosphere/asthenosphere system and in other oceanic systems such as hotspots, mid-ocean ridges and subduction zones.